Altera ALTPLL (Phase-Locked Loop) IP Core - Manual (Page 17)

Cyclone Series PLL Fast PLL Enhanced PLL Left_Right Top_Bottom Device
Yes — — — — Cyclone III
Yes — — — — Cyclone II
No — — — — Cyclone
Spread-Spectrum Clocking
Spread-spectrum technology reduces electromagnetic interference (EMI) in a system. This technology works
by distributing the clock energy over a broad frequency range.
The spread-spectrum clocking feature distributes the fundamental clock frequency energy throughout your
design to minimize energy peaks at specific frequencies. By reducing the spectrum peak amplitudes, the
feature makes your design more likely meets the EMI emission compliance standards, and reduces costs
associated with traditional EMI containment.
The traditional methods for limiting EMI include shielding, filtering, and using multi-layer printed circuit
boards. Multi-layer circuit boards are expensive and are not guaranteed to meet the EMI emission compliance
standards. The use of spread-spectrum technology is simpler and more cost-effective than these other
methods.
To use the spread-spectrum clocking feature, you must set the programmable bandwidth feature to Auto.
Parameter Settings
For devices that support spread-spectrum technology, the parameter settings are located on the Bandwidth/SS
page of the ALTPLL parameter editor.
The following figure shows the Spread Spectrum window.
Figure 10: Spread Spectrum Settings
To enable the spread-spectrum feature, turn on Use spread spectrum feature. Set the desired down spread
percentage, and the modulation frequency. The table in the spread spectrum window lists the detailed
descriptions of the current counter values.
The down spread percentage defines the modulation width or frequency span of the instantaneous output
frequency resulting from the spread spectrum. When you use down spread, the modulation width falls at
or below a specified maximum output frequency. The wider the modulation, the larger the band of
frequencies over which the energy is distributed, and the more reduction is achieved from the peak. For
example, with a down spread percentage of 0.5% and maximum operating frequency of 100 MHz, the output
frequency is swept between 99.5 and 100 MHz.
Altera Corporation ALTPLL (Phase-Locked Loop) IP Core User Guide
Send Feedback
17 Spread-Spectrum Clocking
ug-altpll
2014.08.18